The present invention relates to a focused ion beam scanning apparatus which scans a sample with focused ion beams to form an image in a high resolution, an observation method using focused ion beams, and a processing method using focused ion beams.
The focused ion beam scanning apparatus as an observation means for the observation of grains of wires and the like and a processing means capable of processing a work in an accuracy on the submicron order has been watched with keen interest with the progressive miniaturization of semiconductor devices. When observing grains of a sample, the surface of the sample is scanned with a focused ion beam (sometimes, abbreviated to "FIB") converged in a size on the submicron order, secondary electrons emitted from the surface of the sample are detected, and an enlarged image, i.e., a SIM image (scanning ion microscopic image), of a section of the sample or the structure of the sample is displayed on a display. Generally, the contrast of the SIM image is higher than that of the SEM image (scanning electron microscopic image).
A method of observing the grains of aluminum using an FIB is described in, for example, Proceedings of International Reliability Physics Symposium, pp. 43-52 (1989).
As is generally known, when a sample of a substance subject to oxidation, such as aluminum, is observed in an atmosphere having a low vacuum, an oxide film is formed over the surface of the sample to obstruct the observation of the grains. The oxide film formed over the surface of the sample can be removed by irradiating the same with an FIB. Therefore, the feasibility of the observation of the grains is dependent on the ratio between the rate of removal of the oxide film by irradiation with an FIB and the rate of formation of the oxide film by natural oxidation caused by the atmosphere of the sample chamber.
When observing a comparatively narrow area on a sample in a high magnification by repeating a scanning cycle at a comparatively short period, the grains can be comparatively satisfactorily observed because the succeeding scanning cycle is carried out before a new oxide film is formed after an old oxide film has been removed in the preceding scanning cycle. In the following description, "oxide film" refers to an oxide film including an oxygen adsorption layer. However, when observing a comparatively wide area on the sample in a low magnification by repeating a scanning cycle, the oxide film removing effect of the beam is diminished because the dwell time of the beam for unit area is shorter than that in the observation in a high magnification and, consequently, it is difficult to achieve satisfactory observation of the grains.
It is desirable, from the view point of observation, to obtain a SIM image of a wide area in a high resolution by using a very narrow primary beam. Although it is desirable for forming such a desirable SIM image (1) to increase the current density of the FIB or (2) to raise the vacuum level of the sample chamber, the means (1) has difficulties in forming a very narrow beam having a high current density, and the means (2) increases the cost of the apparatus because a sample stage, a vacuum pump and such suitable for establishing an ultra-high vacuum are expensive and a long time necessary for creating a high vacuum reduces the operation rate of the apparatus.